Exhaust gas discharged from a diesel engine contains, as a main component, a large amount of fine particles. The fine particles, also referred to as diesel particulate matter (DPM), include diesel soot (that is, carbon particles) and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, DPM can pose serious environmental and health risks.
Accordingly, diesel particulate filters (DPFs) have been devised for collecting the particulate matter. The DPFs require regular regeneration wherein the collected particulates are removed (for example, by burning) to maintain the collection capacity of the filter and reduce pressure loss due to clogging. One example approach for filter regeneration is shown by Ichikawa et al. in U.S. Pat. No. 5,595,581. Herein, a pair of filters are used to remove fine particles from exhaust gas during a collection operation. During a filter regeneration operation, blow-by gas is intermittently blown through one of the filters to purge the collected fine particles. The fine particles are pneumatically conveyed to a collector tank wherefrom they are disposed of by burning.
However, the inventors herein have recognized several potential issues with such an approach. As one example, there may not be sufficient blow-by gas to purge one of the filters before the other filter reaches its maximum collection capacity. As another example, the back pressure from the exhaust gas may reduce the purging efficacy of the blow-by gas during the regeneration operation. Either situation may lead to incomplete filter regeneration and degraded collection performance on subsequent collection operations. As yet another example, the approach shown by Ichikawa requires accessory components such as a collector tank and an associated electric heater or burner. Thus, regular maintenance of the accessory components may be necessitated to maintain the performance of the filter apparatus.
Thus, in one example, some of the above issues may be addressed by a method of operating a particulate matter retaining system coupled to an engine exhaust and an engine intake, the particulate matter retaining system comprising at least a first and a second filter, the method comprising, operating in a first mode with the first filter storing particulate matter and the second filter releasing stored particulate matter, with exhaust gas flowing in a first direction through the filters, where flowing in the first direction comprises flowing through the first filter before flowing through the second filter, and where during the first mode, at least some tailpipe gas is drawn from between the first and second filter for expulsion to the atmosphere. The method may further comprise operating in a second mode with the first filter releasing stored particulate matter and the second filter storing particulate matter, with the exhaust gas flowing in a second, opposite, direction through the filters, where flowing in the second direction comprises flowing through the second filter before flowing through the first filter, and where during the second mode, at least some tailpipe gas is drawn from between the first and second filter for expulsion to the atmosphere. In this way, in a given mode, it is possible to coordinate particulate matter storing and release, while flowing exhaust gas through the particulate matter retaining system in a single direction. As such, this concerted action may prevent heavy soot build-up and related pressure issues in the filters. Further, in this way, the direction of exhaust gas flow through the system may be effectively reversed, thereby reversing the particulate matter storing and releasing functions of the filters. By drawing at least some tailpipe exhaust gas from between the filters, specifically after the particulate matter storing filter and before the particulate matter releasing filter, the particulate matter content of exhaust that is expelled to the atmosphere may be substantially reduced. In this way, the quality of emissions from an engine with a particulate matter retaining system may be improved without requiring complicated system maintenance and regeneration.
In one example, during the first mode, gas exiting the second filter may be routed to the engine intake. Similarly, during the second mode, gas exiting the first filter may be routed to the engine intake. The direction and rate of exhaust flow through each of the filters may be adjusted through control valves coupled to the respective filters. In this way, particulates collected in a diesel particulate filter may be routed to an engine intake, thereby reducing the need for filter regeneration, and regeneration related accessory components (such as a collector tank and soot burner, although such regeneration may still be used in some examples, if desired). By using the exhaust gas flow to direct stored particulates into an engine intake, degraded filter purging related to insufficient blow-by gas may also be averted. Furthermore, by reducing the need for actively regenerating the filters using escalated exhaust temperatures, such as may be induced by downpipe fuel injections, the fuel economy of the engine may be improved.
In either operating mode, that is, during the first and second modes, at least some tailpipe gas may be drawn from between the filters for optional further processing in downstream catalytic converters (such as NOx reducing SCR catalysts) and/or for expulsion to the atmosphere. The remaining portion of the gases (including the released particulates) may be routed to the engine intake, specifically to the engine cylinders, for subsequent combustion, through an EGR line. A regulatory valve and/or an EGR valve may determine the amount of tailpipe gas drawn from between the filters and the amount of exhaust recycled to the intake manifold via the EGR line based on engine operating conditions. These conditions may include, for example, a desired EGR rate, the air-fuel ratio, engine speed/load, the exhaust temperature, etc. The PM retaining system may be sequentially operated between the first and second operating modes. In one example, a controller may operate the filters in the first mode for a predetermined time interval, following which the operating mode may be alternated. Similarly, after operating the filters in the second mode for the predetermined time interval, the operating mode may be switched back to the first mode. As one example, each operating mode may only last one hour such that each filter may only have a half maximal soot load at the end of each operation. In other examples, each operating mode may last a different time interval based on different engine operating conditions, where the interval is determined by factors such as a DPF soot load (for example, based on the DPF backpressure), engine soot model, the output from an integrating soot sensor (for example, a sensor that accurately measures the actual PM load in the filters), filter temperature, etc.
While the example suggests operating the PM retaining system sequentially between the first and second operating modes based on a time interval, in other embodiments, operation between the modes may be alternated responsive to a DPF backpressure (for example, a threshold pressure), an engine soot model, the output of an integrating soot sensor capable of accurately measuring the actual PM load in the filters (for example, a threshold PM load), etc.
Further still, the PM retaining system may be operated in a third mode in response to specific engine operating conditions, such as during a peak power and/or high load output. During the third mode, both filters may be storing particulate matter. Herein, the valves may be adjusted such that the exhaust may be directed through both filters before being vented to the atmosphere.
In this way, by alternating operating modes at frequent intervals such that high soot loads are avoided in the filters, clogging of the filters and related pressure drop issues may be reduced. By releasing the collected particulates into the engine intake, regeneration-related components such as collector tanks (for collecting the particulates) and electric burners (for burning the particulates) may be avoided. Furthermore, by reducing the need for active regeneration of the DPFs, wherein an additional amount of fuel is used to increase the temperature of the DPF and burn off the stored soot, over-temperature related component degradation may be reduced while also improving the fuel economy of the vehicle. Further still, by recovering some energy from burning the particulates in the engine intake, engine fuel efficiency may be significantly improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.